Hemoglobin A1c (HbA1c) is used by Endocrinologists as an indication of blood sugar control in known diabetics. It is also used as a screening test for diagnosis of diabetes, and is the predominant screening test for diabetes in patients who have cystic fibrosis. The theory behind the use of HbA1c is that the life span of human red blood cells is approximately three months; thus the percent of glucose bound to the red blood cell (HbA1c) gives an indirect measurement of blood sugar control for the preceding three months. Any patient whose red blood cell turnover rate is accelerated would have a falsely lower HbA1c. Patients with obstructive pulmonary disease and persons who have accelerated metabolic rates are known to have rapid rates of red blood cell turnover. Cystic fibrosis (CF) patients have both obstructive pulmonary disease and accelerated metabolic rates. Their red blood cell turnover rate is unknown. The hypothesis of this study is that red blood cell senescense occurs earlier in CF patients, and therefore HbA1c measurements cannot accurately be used as a diagnostic screen of diabetes in these patients. This is a pilot study of red blood cell half-life in 10 CF patients who have moderate obstructive lung disease. Radioactive decay of Chromium-51 (red blood cell turnover rate) obtained from these 10 subjects will be compared to known, published normal values.